Arrhythmias remain a major cause of morbidity and mortality. Brugada syndrome is a rare, autosomal dominant, male predominant form of idiopathic ventricular fibrillation characterized by a right bundle branch block pattern and ST elevation in the right precordial leads of the surface EKG. The only effective therapy is an implantable cardioverter-defibrillator. Mutations of the cardiac Na+ channel SCN5A cause ~20% of cases of Brugada syndrome by decreasing inward Na+ current, and Na+ channel blockers such as procainamide exacerbate the EKG findings. The genetic basis for most remaining Brugada syndrome patients is unknown. During the initial periods of this project, we identified a large family with Brugada syndrome characterized by progressive conduction disease, age- and sex-dependent penetrance, and minimal response to the Na+ channel blocker procainamide. Linkage was identified to a ~1 cM region on chromosome 3p24 (max LOD score >4.0) and SCN5A was excluded (LOD score <-2). Direct sequencing of candidates in the region identified an alanine to valine substitution in a conserved amino acid (A280V) in exon 6 of the glycerol-3- phosphate dehydrogenase 1-like gene (GPD1-L, KIAA0089). The mutation was present in all affected individuals and was absent in >200 unaffected controls of mixed racial background. Northern and Western blot analysis confirmed expression in the heart. Whole cell patch clamp studies of a stably transfected HEK cell line expressing SCN5A showed an ~60% reduction in peak Na current in cells transfected with the A280V mutant compared to the wild type GPD1-L (p=0.01). Confocal microscopy showed expression of the wild type but not the mutant GPD1-L protein on the plasma membrane, along with reduced SCN5A in the membrane of cells expressing the A280V mutant. NADH is increased in cells expressing A280V GPD1-L, and QRS duration on the surface EKG is prolonged in transgenic mice overexpressing A280V GPD1-L. In this competing renewal, we will test the hypothesis that GPD1-L is a novel ion channel modulator and that mutations of GPD1-L decrease Na+ current by altering intracellular NAD+/NADH levels. We will 1) define the mechanisms by which mutant GPD1-L alters ion channel trafficking and current in-vitro using cell lines and neonatal rat ventricular myocytes;2) identify downstream mechanisms by which GPD1-L affects ion channel trafficking, and 3) study transgenic and gene-targeted mouse models to confirm its physiological role. Over 250,000 people in the United States die suddenly each year. Most sudden death occurs during heart attacks or in patients with weak hearts from prior damage, but we have no good ways to predict who is at highest risk of deaths and no drug therapies to prevent sudden death. In this proposal, we will study a family with an inherited genetic form of sudden death with the hope that we can develop new treatments for the more common causes of this devastating and unpredictable condition.